Vitamins are a vital part of a healthy diet. The recommended dietary allowance (RDA)—the amount most healthy people need each day to remain healthy—has been determined for most vitamins. A safe upper limit (tolerable upper intake level) has been determined for some vitamins. Intake above this limit increases the risk of a harmful effect (toxicity).
Consuming too little of a vitamin can cause a nutritional disorder. However, people who eat a variety of foods are unlikely to develop most vitamin deficiencies. Deficiency of vitamin D is an exception. Vitamin D deficiency Vitamin D Deficiency Vitamin D deficiency is most commonly caused by a lack of exposure to sunlight. Some disorders can also cause the deficiency. The most common cause is lack of exposure to sunlight, usually when... read more is common among certain groups of people (such as older people) even if they eat a variety of foods. For other vitamins, a deficiency can develop if people follow a restrictive diet that does not contain enough of a particular vitamin. For example, vegans, who consume no animal products, may become deficient in vitamin B12 Vitamin B12 Deficiency Vitamin B12 deficiency can occur in vegans who do not take supplements or as a result of an absorption disorder. Anemia develops, causing paleness, weakness, fatigue, and, if severe, shortness... read more , which is available in animal products. Deficiency of the B vitamins biotin or pantothenic acid almost never occurs. People at high risk for vitamin deficiencies (for example, people who had bariatric surgery, are on hemodialysis, or have alcohol use disorder) may benefit from a daily multivitamin.
Consuming large amounts (megadoses) of certain vitamins (usually as supplements) without medical supervision may also have harmful effects.
Vitamins are called essential micronutrients because the body requires them but only in small amounts.
The body does not store most vitamins. Deficiencies of these vitamins usually develop in weeks to months. Therefore, people must consume them regularly.
Vitamins A, B12, and D are stored in significant amounts, mainly in the liver. Vitamins A and D are also stored in fat cells. Deficiencies of these vitamins take more than a year to develop.
Because many people eat irregularly or do not eat a variety of foods, they may not get enough of some vitamins from foods alone. If they do not get enough, the risk of certain cancers or other disorders may be increased. People may then take a multivitamin. However, for most people, taking multivitamins does not appear to reduce risk of developing cancer or heart or blood vessel (cardiovascular) disorders.
Some vitamins are fat soluble. Other vitamins are water soluble. The difference between fat and water soluble affects nutrition in several ways.
Fat-soluble vitamins
Fat-soluble vitamins dissolve in fats (lipids) and include
Vitamin A
Vitamin D
Vitamin E
Vitamin K
Fat-soluble vitamins are stored in the liver and in fatty tissues. If too much of the fat-soluble vitamins A or D are consumed, they can accumulate and may have harmful effects.
Because fats in foods help the body absorb fat-soluble vitamins, a low-fat diet may result in a deficiency. Some disorders, called malabsorption disorders Overview of Malabsorption Malabsorption syndrome refers to a number of disorders in which nutrients from food are not absorbed properly in the small intestine. Certain disorders, infections, and surgical procedures can... read more , interfere with absorption of fats and thus of fat-soluble vitamins. Some drugs, such as mineral oil, have the same effect. Fat-soluble vitamins dissolve in mineral oil, which the body does not absorb. So when people take mineral oil (for example, as a laxative), it carries these vitamins unabsorbed out of the body.
Cooking does not destroy fat-soluble vitamins.
Water-soluble vitamins
Water-soluble vitamins dissolve in water and include
B vitamins
Vitamin C
B vitamins include biotin, folate (folic acid), niacin, pantothenic acid, riboflavin (vitamin B2), thiamin (vitamin B1), vitamin B6 (pyridoxine), and vitamin B12 (cobalamins).
Water-soluble vitamins are eliminated in urine and tend to be eliminated from the body more quickly than fat-soluble vitamins. Water-soluble vitamins are more likely to be destroyed when food is stored and prepared. The following can help prevent the loss of these vitamins:
Refrigerating fresh produce
Storing milk and grains out of strong light
Using the cooking water from vegetables to prepare soups
Causes
Disorders that impair the intestine’s absorption of food (called malabsorption disorders Overview of Malabsorption Malabsorption syndrome refers to a number of disorders in which nutrients from food are not absorbed properly in the small intestine. Certain disorders, infections, and surgical procedures can... read more ) can cause vitamin deficiencies.
Some disorders impair the absorption of fats. These disorders can reduce the absorption of fat-soluble vitamins—A, D, E, and K—and increase the risk of a deficiency. Such disorders include chronic diarrhea Diarrhea in Adults Diarrhea is an increase in the volume, wateriness, or frequency of bowel movements. (See also Diarrhea in Children.) The frequency of bowel movements alone is not the defining feature of diarrhea... read more , Crohn disease Crohn Disease Crohn disease is an inflammatory bowel disease where chronic inflammation typically involves the lower part of the small intestine, the large intestine, or both and may affect any part of the... read more 
, cystic fibrosis Cystic Fibrosis (CF) Cystic fibrosis is a hereditary disease that causes certain glands to produce abnormally thick secretions, resulting in tissue and organ damage, especially in the lungs and the digestive tract... read more , certain pancreatic disorders, and blockage of the bile ducts Overview of Gallbladder and Bile Duct Disorders 
.
Some types of weight-loss (bariatric) surgery Bariatric Surgery Bariatric (weight-loss) surgery alters the stomach, intestine, or both to produce weight loss. In the United States, about 160,000 people have bariatric surgery each year. This number accounts... read more can also interfere with absorption of vitamins.
Liver disorders Overview of Liver Disease Liver disease can manifest in many different ways. Characteristic manifestations include Jaundice (a yellowish discoloration of the skin and whites of the eyes) Cholestasis (reduction or stoppage... read more and alcoholism Alcohol-Related Liver Disease Alcohol-related liver disease is liver damage caused by drinking too much alcohol for a long time. In general, the amount of alcohol consumed (how much, how often, and for how long) determines... read more 
can interfere with the processing (metabolism) or storage of vitamins.
In a few people, hereditary disorders impair the way the body handles vitamins and thus cause a deficiency.
If people must be fed intravenously for a long time or if the formula used lacks the needed nutrients, people may develop a vitamin (or mineral) deficiency.
Drugs can also contribute to deficiency of a vitamin. They may interfere with absorption, metabolism, or storage of a vitamin.
